The present invention relates to an electrical cable, in particular for medium- or high-voltage power transmission or distribution, having a semiconductive water-blocking expanded layer. In the present description, the term xe2x80x9cmedium voltage xe2x80x9d is understood to refer to a voltage of between about 1 kV and about 30 kV, while the term xe2x80x9chigh voltage xe2x80x9d is understood to refer to voltages above 30 kV.
Cables for medium- or high-voltage power transmission or distribution generally consist of a metal conductor coated with a first inner semiconductive layer, an insulating layer and an outer semiconductive layer. For some uses, in particular when it needs to be watertight with respect to the exterior, the cable is enclosed inside a metal shield, usually an aluminium or copper shield, consisting of a continuous tube or a metal sheet shaped into a tube and welded or sealed so as to be watertight.
During production, installation or use, breakages and piercings can occur in the metal shield, which allow penetration of moisture or even water into the cable core, with formation of electrochemical trees in the insulation layer, which can cause insulation failure.
A possible solution to this problem is provided in U.S. Pat. No. 4,145,567. A high-voltage cable is described therein having, around the outside of the outer semiconductive layer, a compressible layer of a foamed plastic material which should prevent external moisture from reaching the insulation layer, thus avoiding formation of electrochemical trees. According to that disclosure, the metal shield preferably maintains some pressure against the compressible layer so that no air or other fluid can travel along the interface between the compressible layer and the metal shield. As further insurance against passage of fluid along the cable, the metal shield can be bonded to the compressible layer. The compressible layer is preferably semiconducting.
Ruptures in the metal shield may be caused by the thermal cycles to which the cable is subject as a result of the daily variations in the intensity of the transported current, with corresponding variations in the cable temperature of between room temperature and the maximum operating temperature (for example between 20xc2x0 C. and 90xc2x0 C.). These thermal cycles cause dilation and subsequent contraction of the coating layers of the cable, with consequent radial forces exerted on the metal shield. The metal shield can thus suffer mechanical deformations with formation of empty spaces between the shield and the outer semiconductive layer, which may give rise to non-uniformity in the electrical field. At the utmost, these deformations can lead to rupture of the shield, particularly when it is welded or attached by means of sealing, and hence to complete loss of functionality of the shield.
A possible solution to this problem is provided in U.S. Pat. No. 5,281,757, where the metal shield is free to move with respect to the adjacent layers and has the overlapping edge portions bonded together by an adhesive which allows the overlapping edge portions to move relative to each other during the thermal cycling of the cable. A cushioning layer as that disclosed in the above-mentioned U.S. Pat. No. 4,145,567 may be applied between the metal shield and the cable core. If desired, the cushioning layer may be a water swellable tape or a water swellable powder instead of a foamed plastic material.
According to the Applicant""s experience, cable designs such as those described in U.S. Pat. Nos. 4,145,567 and 5,281,757 are not completely satisfactory. Firstly, the presence of a compressible layer between metal shield and cable core as disclosed in U.S. Pat. No. 4,145,567 is not sufficient to effectively avoid penetration and propagation of moisture or water along the cable. In fact, to obtain an effective water-blocking effect, in U.S. Pat. No. 5,281,757 it is suggested to use, instead of the compressible layer, a water-swellable tape or powder. However, the introduction of a water-swellable material under the metal shield would cause serious electrical problems. In fact, the metal shield, in addition to constituting a barrier against penetration of water and/or moisture, exerts important electrical functions and needs to be in electrical contact with the outer semiconductive layer. A first function of the metal shield is indeed to create a uniform radial electric field inside the cable and, simultaneously, to cancel out the electric field outside the cable. A further function is to support short-circuit currents.
The presence of an insulating material such as a water-swellable material under the metal shield cannot ensure electrical continuity between the cable core and the metal shield. Moreover, from the point of view of production and handling, the use of water-swellable tapes or of free water-swellable powders has many drawbacks. Particularly, the use of a water-swellable tape involves an appreciable increase in costs and a decrease in productivity, since these tapes are expensive and imply the addition of a wrapping stage to the cable production process. On the other hand, the presence of free-flowing water-swellable powders makes production and installation of the cable quite cumbersome.
Finally, cables are known in the art which are designed to attenuate the effect of the thermal cycles on the metal shield and at the same time to avoid propagation of moisture and/or water along the cable. These cable are provided with an outer semiconductive layer with V-shaped longitudinal grooves which are filled with a water-swellable material in the form of powder. The V-shaped geometry should ensure electrical contact between the semiconductive layer and the metal shield, on the one hand, and should assist the elastic recovery of the thermal dilations by the material which makes up the semiconductive layer, on the other hand.
However, producing these longitudinal grooves involves the use of a semiconductive layer of high thickness (about 2 mm or more), thereby increasing the cost and the overall weight of the cable. In addition, the desired geometry of the semiconductive layer is generally achieved by means of a precise process of extrusion in which appropriately designed dies are used. On the basis of the Applicant""s experience, the formation of grooves of irregular geometry is, in practical terms, inevitable during such an extrusion process. These geometrical irregularities can give rise to a non-uniform distribution of the pressure exerted on the metal shield and thus prevent the semiconductive layer from correctly carrying out its function of elastic absorption of the radial forces.
Therefore, the cables according to the above prior art cannot effectively address both the problem of avoiding penetration and propagation of moisture and/or water inside the cable core, and the problem of possible deformations or breakages of the metal shield due to the cable thermal cycles, while maintaining a proper electrical contact between metal shield and cable core.